Although there is significant evidence that 2,3,7,8- tetrachlorodibenzodioxin (TCDD) alters lymphocyte function in mice, there have been very few studies which examine the effects of TCDD on human lymphocytes. While there have been a number of inadvertent exposures to TCDD, via industrial accidents or environmental contamination, there have been no consistent immunological defects observed in exposed individuals. Studies in German chemical workers have demonstrated alterations in lymphocytes subpopulations and functional parameters 20 years after exposure to TCDD. As part of a continued effort to assess the health effects of environmental exposures to TCDD it is imperative that we examine endpoints which have the potential to be sensitive predictors of human exposure and to correlate these biomarkers with blood TCDD levels, determine influence of potential confounding variables on biomarker expression, and evaluate inter- and intra-individual variation. Identification of additional biomarkers may aid in identifying individuals or populations at greater risk of developing disease or neoplasia after exposure to dioxins and related compounds. Peripheral blood lymphocytes were obtained from several cohorts of individuals exposed to dioxins occupationally, accidentally, or from low level background environmental exposure. These samples were mitogen-stimulated in culture, alone or in the presence of TCDD, and the cells and culture supernatants harvested after 72 hr. The initial cohort we examined consisted of a group of workers who were employed from 1951-1984 at a chemical plant in Germany that produced organochlorine herbicides and pesticides (including 2,4,5-trichlorophenoxyacetic acid) and opioids. TCDD contamination at this plant was confirmed in 1984 and significant increases in mortality have been observed in a cohort of workers employed at this plant for at least 3 months from 1952-1984. Peripheral blood for lymphocyte isolation and serum dioxin analysis was obtained from 110 individuals in 1992, in collaboration with clinicians at the University of Mainz (Detlev Jung and Johannes Konietzko) and a biostatistician at the German Cancer Research Center (Lutz Elder). In vitro treatment with TCDD resulted in significant decreases in the mitogen-stimulated production of the T Helper 1 type (TH1) cytokines, Interleukin 2 (IL2) and Interferon gamma (IFNgamma) in lymphocyte cultures from this cohort. We believe the alterations inTH1 cytokines are a result of the in vitro exposure as there is limited correlation with standard biomarkers of TCDD exposure such as total TCDD plasma levels or total plasma TEQ. Significant correlations were also seen between individual levels of in vitro TCDD-inducible ethoxyresorufin-O- deethylase activity (a robust measure of cellular response to TCDD exposure) and cytokine secretion (e.g. tumor necrosis factor alpha). In collaboration with Dr. Joseph Haseman, the dataset is undergoing multivariate analysis to examine correlations between immunologic effects and in vivo and in vitro biomarkers of exposure, and to determine the effects of confounding variables such as gender, smoking status, etc. In a similar study, we examined peripheral blood lymphocytes from a second cohort, individuals acutely exposed to TCDD following a chemical plant explosion in Seveso, Italy. As was seen in the other cohort, in vitro treatment with TCDD resulted in a significant decrease in the secretion of IL2 following mitogen stimulation. - TCDD, Cytokines, Immunoglobulins, T helper cells, lymphocytes, Interleukins, ELISA